CHEMICAL SCIENCE

In-situ direct electrolysis of seawater to produce hydrogen, solving the problem of half a century in this field!


On November 30, 2022, the team of Academician Xie Heping of Shenzhen University/Sichuan University made a major original breakthrough in the field of direct hydrogen production from seawater, and published a research result entitled “A membrane-based seawater electrolyzer for hydrogen generation” in the journal Nature.

This study for the first time from the new idea of combining physical mechanics and electrochemistry, established a new principle and technology of phase change migration-driven in-situ direct electrolysis of seawater without desalination, and completely isolated seawater ions to achieve a major original breakthrough in the principle and technology of in-situ direct electrolysis and efficient hydrogen production of seawater without desalination process, no side reaction, and no additional energy consumption (that is, using seawater as pure water and directly producing hydrogen by in-situ electrolysis in seawater).

Academician Xie Heping is the first and corresponding author of this paper, and Professor Shao Zongping is the co-corresponding author.

▌Background introduction

Green zero-carbon hydrogen energy is an important direction for future energy development, with the explosive growth of hydrogen energy, it is expected that by 2060, China’s annual demand for hydrogen will reach 130 million tons, when it will consume about 1.17 billion tons of pure water for electrolysis per year. However, the shortage of fresh water resources will seriously restrict the development of “green hydrogen” technology. The ocean is the largest hydrogen mine on the earth, and asking for water from the sea is an important direction for the future development of hydrogen energy! However, the complex composition of seawater (about 92 chemical elements) has led to many problems and challenges in the production of hydrogen from seawater. Hydrogen production after desalination is currently the most mature technology path for hydrogen production from seawater, and large-scale demonstration projects have been carried out in many countries around the world. However, this type of technology relies heavily on large-scale desalination equipment, and the process flow is complex and occupies a lot of land resources, which further increases the cost of hydrogen production and the difficulty of engineering construction. In the early 70s of last century, some scientists proposed whether seawater could be directly electrolyzed to produce hydrogen? In the past half century, Stanford University in the United States, the National Center for Scientific Research in France, the University of Adelaide in Australia, and the Membrane Materials Science of the Chinese Academy of Sciences have carried out a lot of exploration and research, aiming to solve the problems of chlorine evolution side reaction, calcium and magnesium precipitation, catalyst inactivation and other problems faced by direct electrolysis of seawater hydrogen production. However, so far, there is no breakthrough theory and principle to completely avoid the influence of complex components of seawater on electrolysis hydrogen production, and the principle and technology of large-scale efficient and stable direct electrolysis hydrogen production of seawater is still a blank in the world, which has become a half-century problem in this field.

In this work, Academician Xie Heping proposed a new idea from the combination of physical mechanics and electrochemistry to solve the problems and challenges faced by direct electrolysis of seawater for hydrogen production, thereby creatively creating a new principle and technology of hydrogen production without desalination in situ. By skillfully combining physical and mechanical processes such as molecular diffusion and interfacial phase equilibrium with electrochemical reactions, a theoretical model of direct electrolysis hydrogen production from seawater driven by phase change migration was established, and the influence mechanism of interfacial pressure difference under micron-level air gap pathway on spontaneous phase change mass transfer in seawater was revealed, and a dynamic self-regulating stable electrolysis hydrogen production method with electrochemical reaction and seawater migration was formed, which solved the problem of harmful corrosion, which plagued the field of seawater electrolysis hydrogen production for half a century.

▌The new principle of direct hydrogen production from seawater is constructed

The researchers used the waterproof and breathable layer to construct a micron-scale “gas phase” isolation domain in seawater, and relied on the naturally existing saturated vapor pressure difference between the self-humidifying electrolyte and seawater as the driving force for mass transfer, and realized the spontaneous phase change migration process of water vaporization from the seawater side, diffusion in the membrane to the electrolyte side to absorb liquefaction. The intrinsic hydrophobic effect of the waterproof and breathable layer completely isolates liquid seawater from its impurity components, and only allows seawater to diffuse in the form of water molecules; The high saturated vapor pressure of seawater and the low saturated vapor pressure of the high concentration electrolyte form a driving force, which promotes the transmembrane transport of water molecules to the electrolyte side and liquefaction under the action of electrolyte hydration and absorption. The simultaneous consumption of water by electrolysis hydrogen production further maintains the pressure difference at the membrane interface, thereby inducing continuous replenishment of water from seawater to the electrolyte. The laboratory-scale seawater direct hydrogen production device developed by this new principle operates stably in the seawater of Shenzhen Bay for more than 72h, the Faraday efficiency is nearly 100%, the ion content in the electrolyte is four times lower than the corresponding ions in seawater, and its electrolysis energy consumption is comparable to that of industrial alkaline electrolyzed water, which verifies the feasibility and excellent performance of the principle of direct electrolysis of seawater driven by “liquid-gas-liquid” phase change migration.

Figure 1: Principle and excellent performance of hydrogen production by in-situ direct electrolysis of seawater without desalination

The critical role of micron-sized gas-phase isolation domains

The low surface energy, porous PTFE-based waterproof and breathable layer forms a superhydrophobic barrier domain interface, and its ion barrier effect shows a high degree of harmonization, which is not significantly affected by time changes and membrane pore size. The immersion gas phase channel formed by this isolation domain also provides efficient directional mass transfer efficiency, increasing the water migration rate by two orders of magnitude over micron transport distances compared to direct water acquisition over long distances above the sea surface (above centimeters). At the same time, the results show that the migration rate of water molecules in the air gap channel can be further flexibly adjusted by factors such as mass transfer area, membrane pore size, and path length. In addition, the researchers also investigated its antifouling performance, and the SEM results showed that there was almost no membrane pore clogging in the real seawater environment for 15 days.

Figure 2: Ion blocking effect and directional transport effect in micron-sized vapor phase isolation domain

▌Self-humidifying electrolyte provides the core driving force

Self-humidification with low saturated vapor pressure, high ionic conductivity and wide electrochemical window provides a continuous driving force for the spontaneous migration of water, and plays an important role in the direct hydrogen production process driven by phase change migration. Interestingly, a large number of common electrolytes have the above properties, such as KOH. The researchers further revealed the mechanism of action of water migration dynamics and its influence on the electrochemical properties of the system. When the electrolysis process was not loaded, the KOH electrolyte concentration decreased from 100wt% to 13.3wt% within 90min. In this process, the driving force of the interfacial vapor pressure difference caused by the decrease in electrolyte concentration continues to shrink, resulting in a gradual decrease in the water migration rate. In addition, as the electrolyte concentration decreases, its conductivity also changes significantly by increasing first and then decreasing, with a peak at a concentration of 30 wt%. This provides a strong basis for the selection of electrolyte concentration after balancing electrolytic performance with water migration rate.

Figure 3: Balance selection of self-humidifying electrolyte between water migration rate and electrolytic hydrogen production performance

Exploration of stable and efficient direct hydrogen production mechanism from seawater

The researchers further established a theoretical model of direct electrolysis hydrogen production from seawater driven by phase change migration through Darcy’s Law and Faraday’s Law to reveal the theoretical upper limit and equilibrium mechanism of continuous hydrogen production from seawater. Model analysis showed that the system equilibrium was mainly affected by the interfacial vapor pressure difference caused by electrolyte concentration. When the water migration rate coincides with the electrolysis rate, the interfacial vapor pressure difference is always constant, so the system is in equilibrium. Interestingly, the hydrogen production from seawater driven by phase change migration is a self-regulating dynamic equilibrium system: when the initial electrolysis rate is greater than the water migration rate, the electrolyte concentration will gradually increase and cause the interfacial vapor pressure difference to increase, which in turn leads to an increase in the water migration rate to match the electrolysis rate; When the initial electrolysis rate is less than the water migration rate, the water migration rate decreases with the interface vapor pressure difference to match the low electrolysis rate. The researchers further explored the upper limit of the maximum theoretical electrolytic current satisfied by the water migration rate, and the results showed that 1g KOH can meet the theoretical current of 5.8~38.8 A, which has great potential.

Figure 4: Continuous and efficient hydrogen production and self-regulating mechanism

Large-scale, long-term stable direct hydrogen production from seawater

Researchers independently developed 386 LH-1 seawater direct hydrogen production technology and equipment. The compact equipment (82 cm × 62 cm × 70.5 cm) consists of 11 electrolytic units. The large-scale equipment operated stably for up to 3200 h at a current density of 250 mA cm-2 in the seawater of Shenzhen Bay (without pretreatment), and the electrolysis energy consumption was about 5.0 kWh Nm−3 H2. During the electrolysis process, the content of impurity ions in the electrolyte is always four orders of magnitude lower than that of the corresponding ions in seawater, proving that there is almost no membrane wetting and seawater permeation in long-term operation. In addition, SEM reveals the morphological characteristics of the anode catalyst under long-term operation, and the results show that the single electrolyte environment provided by this strategy makes the catalyst almost corrosive. Due to its compact structure design, simple system engineering and excellent electrolytic performance, the system has great advantages in the energy construction of future ecological floating islands.

Figure 5: Large-scale seawater direct hydrogen production technology and equipment and long-term stability in Shenzhen Bay seawater

▌The diversity and scalability of the new strategy of direct hydrogen production from seawater

The researchers further developed acidic and basic solid gel electrolytes to show that the phase change migration strategy is suitable for different electrolyte materials and is expected to accompany the iterative development of PEM and AEM electrolysis hydrogen production technologies. At the same time, the researchers used Mo-Ni3S2/NF anode catalyst to conduct long-term tests, indicating that the development of high-performance catalysts in alkaline environment is one of the means of system optimization. In addition, this principle technology can also be explored and promoted to the direct in-situ hydrogen production of diversified water resources (such as river water, waste water, salt lakes, etc.), providing new ideas for multi-effect utilization for resource enrichment and energy production.

▌Summary and outlook

“Seawater without desalination in-situ direct electrolysis hydrogen production” integrates multidisciplinary theory, is an original principle and technological breakthrough independently developed by China, and opens up a new path for global seawater hydrogen production (applied for patent: CN2021110197054; CN114481164A; CN2022110704439; CN2022110748884; PCT/CN2022/128225); This technology can integrate “offshore wind power and other renewable energy utilization-seawater resource utilization-hydrogen energy production”, and is expected to form a new mode of in-situ seawater direct electrolysis hydrogen production without desalination, no additional catalyst engineering, no seawater transportation, and no pollution treatment, and truly transform inexhaustible “seawater resources” into “seawater energy”! In the future, this technology can build an integrated in-situ seawater hydrogen production plant combined with offshore renewable energy, which is expected to become the key to the large-scale development of renewable power in the deep sea, and accelerate the formation of China’s original “marine green hydrogen” global emerging strategic industry with multi-energy complementarity! (Source: Science Network)

Related paper information:https://doi.org/10.1038/s41586-022-05379-5



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